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450 million years. but have only experienced large urban areas for a few hundred years. scientists at the university of miami wanted to find out if sharks are attracted to or repelled by big cities. >> what we were trying to do is track these sharks to see if they would be avoiding areas really close to the city that were polluted, lots of boat traffic, lots of human disturbance. and we really thought they would. >> reporter: using acoustic technology, researchers monitored the moms of bull sharks, nurse sharks, and great hammerhead sharks off the coast of miami for several years. >> essentially what we did is placed these transmitters, small little tags on the sharks. they're about the size of a aa battery. they would emit an ultrasonic sound. >> reporter: the sound included a code that identifies each shark. they then place listening